This invention relates to a method and an apparatus for reduction of flow resistance in pipes and ducts where a fluid or a powder is flowing in single or multiphase. In the method, the flow resistance is reduced by applying an electrical field on the pipe or duct wall. Also, the strength of the field is regulated according to measurements of the flow regime before and after the unit which exposes the fluid or powder to the electrical field. The fluid may be a pure fluid, colloidal fluid or contain inclusions in the form of particles.
Many important industrial processes and community works involve transport of fluids in pipes. Examples are among others, supply of water to hydro-electric power stations, waterworks, water purification plants, and sewage treatment el. purification plants, or the distribution net for district heating plants, transport of oil and gas in pipes, and process-lines in process chemistry, food industry and petrochemical industry.
A common problem connected with all forms of transport of fluids in pipes and ducts, is the loss of fluid pressure due to the flow resistance. This pressure loss causes loss of energy for all processes which includes pipe transport of fluids. For larger transport distances, this may become an important economical factor since the pressure loss must be compensated by regeneration of the fluid pressure by one or several pumping stations. Thus, it is both from an environmental and economical point of view of interest to reduce the flow resistance.
It has been known since the nineteenth century that by imposing a magnetic field to water flowing in a pipe, the formation of calcareous deposits on the inner walls of the pipe can be reduced and/or avoided. This effect is thoroughly discussed in American Petroleum Institute Publication 960, September 1985. Although there are similarities between this effect and the present invention, both the aim and means are sufficiently distant that this has only restricted resemblance with the present invention.
The flow velocity of a fluid which is flowing through a pipe/duct will vary along the cross section of the pipe/duct. The highest velocity is achieved in the middle and the lowest at the boundary between the fluid and the wall of the pipe/duct. Typical velocity profiles for laminar and turbulent flow in pipes [1] is given in FIG. 1.
The shape of the velocity profile is determined by the Reynolds number and the friction factor for the fluid flow. The Reynolds number is determined by the density of the fluid, dynamic viscosity, average flow velocity, and the diameter of the pipe/duct. If the Reynolds number is less than 2300 the flow becomes laminar (parabola shaped velocity profile) and turbulent if it is above 2300. The friction factor is determined by the roughness of the pipe/duct wall and the Reynolds number. The roughness is a complex quantity which depends on parameters such as the shape of the pipe/duct wall, size, physical character of the surface, and electrical conditions [2]. All these parameters tend towards reducing the flow velocity. The roughness is normally determined by measurements of fluid pressure loss. The roughness as a function of the Reynolds number and friction factor for a number of materials is given as a Moody diagram [2] in FIG. 2.
It is also known that when a piece of metal is submerged in water, some of the metal will be dissolved as positive metal ions and the metal piece becomes negatively charged. Due to electromagnetic attraction, a layer of positively charged metal ions, hydrogen ions (dependent on pH), other positively charged ions present in the water, an d polar molecules with the positive end facing the metal piece will be formed [3]. An illustration if this layer is given in FIG. 3. A voltage which and can be measured in relation to a standard reference cell (for instance a standard calomel electrode, SCE) is thus forming across this layer, and is named the corrosion potential [4]. The layer, which is called the electrical double layer, has a thickness in the order of 10xe2x88x929m. Although the potential across the layer is in the order of IV, the electrical field is very large in the order of 109V/m [3].
To maintain the corrosion potential, a small current of ions from the solution to the electrode has to occur, and a concentration gradient will then be established. This concentration gradient is called the diffusion layer, and has a thickness of about 0.1 mm. The thickness depends on the stirring rate, or flow velocity. The higher stirring rate, or flow velocity the thinner the diffusion layer will be, The thinner diffusion layer, the higher current of ions to the electrode, and hence, the higher corrosion potential [3].
EP 0 661 237 A1 discloses a method for preventing deposition of calcium and magnesium scale on pipe walls by imposing a DC electric potential for ionisation of the fluid. However, ionisation of the fluid will enlarge the corrosion potential and this method is therefore not relevant for this invention.
U.S. Pat. No. 5 480 563 discloses a method for removing electrostatic charges which builds up in highly resistivity liquids without contacting the liquid in order to avoid polluting it. An example of a such liquid is extreme pure water employed in the manufacture of semiconductor devices and liquid crystal devises. It is known that such water can be charged up to 1000V after passing a teflon-based pipe, and may be damaging to the device under production. The solution is to employ electrodes covered with a thin inert layer that allows tunneling electrons to pass into the liquid. However, the large a potentials needed to perform this task will inevitably increase the corrosion potential and thereby the flow resistance, and is therefore not relevant for this invention.
The idea which the present invention is based on is that the gathering of ions and polar molecules at the fluid-wall boundary due to the corrosion potential will increase the friction factor and thus slow down the fluid flow.
A general object of this invention is,to provide a method which prevents the increase of the friction factor due to the corrosion potential present between a flowing fluid and the wall of a pipe/duct, and thus to reduce the loss of pressure for fluids flowing:,in a pipe/duct.
Another object of this invention is to provide an apparatus for carrying out the method.
The general idea of this invention is that the build-up of ions at the fluid-solid boundary can be counteracted by imposing a DC electric potential on the pipe/duct wall. The magnitude of the potential should be such that it exactly balances the build-up of electrical charges on the wall. Then the electromagnetic force that attracted the ions and polar molecules will diminish, and the ions and polar molecules may freely follow the flowing fluid. In other words, the electrical contribution to the friction factor becomes zero.
A reversed situation occurs if the imposed electric potential becomes larger than the build-up of electrical charges. Then there will be a build-up of electrical charges with opposite values on the pipe/duct wall, and ions (with opposite charges) and polar molecules (with the opposite end facing the wall) will adhere to the wall and thus increase the friction factor. It is therefore important to find the magnitude of the imposed electrical potential which balances the build-up of electrical charges.
The object of the invention is achieved for instance by an embodiment as shown schematically in FIG. 4. The Figure shows a pipe in which a fluid is flowing in the direction of the arrow. A short part of the pipe wall is electrically insulated form the rest of the pipe wall in both ends. The inner diameter of the pipe and the insulated part of the pipe should be equal in order not to disturb or to introduce Unnecessary pressure losses in the fluid flow. A DC electric potential generator is connected with one polarity to the insulated pipe part and the other polarity to the pipe downstream of the insulated part or to another insulated part of the pipe downstream the first insulated part This insulated part is similar to the first insulated part. The DC electric potential generator is continuously regulated by a regulating unit which reacts to measurements of the fluid quality anywhere upstream of the part of the pipe which is exposed to the electrical potential. This ensures that the system can impose the correct value of the electrical potential regardless of which fluids employed and of eventual changes in the flow
By the quality of the fluid we mean quantities such as fluid flow velocity, corrosion potential for the actual pipe, pH, concentration of specific ions, electrical conductivity, pressure, and fluid temperature. The regulating unit may employ some or all of these measured quantities when calculating the correct value of the imposed electrical potential, The regulating unit may be a standard computer unit which receives the measured data and which can control the DC electric potential generator.